The use in tyres of a viscous sealant layer generally arranged in the inner cavity of the tyre in contact with the air or covered by a protective rubber or polymer layer has been known for some time. In particular, the sealant layer is generally arranged on the central zone of the inner cavity in the area of the tread strip.
The object of the sealant layer is to surround and adhere to the object that has penetrated the tread, thus preventing the outflow of air from the tyre by instant “sealing”. Furthermore, if the penetrated object comes out of the tread, the material of the sealant layer will be arranged to occupy the hole left by the object sealing the same.
Part of the research into tyres concentrates on improving the effectiveness of the sealant layer with particular attention to its rheological characteristics. In fact, the viscosity of the sealant layer must guarantee both the sealing action with respect to the penetrated object and with respect to any hole as described above, and its stability in the inner cavity regardless of the static or dynamic conditions of the tyre.
As may appear evident to a person skilled in the art, the sealant layer, being constantly in contact with the pressurized air, is one of the parts of the tyre most exposed to oxidative degradation, consequently compromising its sealing capacity.
The need was therefore felt for a sealant layer, with technical characteristics such as to guarantee maintenance of its sealant properties throughout the life of the tyre.
Lignin is an organic substance which binds the cells and the fibres that make up wood and the lignified elements of plants. After cellulose, it is the most abundant renewable source of carbon on the earth. Although it is not possible to define the precise structure of lignin as a chemical molecule, lignin can be defined as a dendritic polymer based on the following three phenylpropane units: -p-coumaryl alcohol; -coniferyl alcohol (4-hydroxy-3-methoxycinnamyl alcohol); -sinapyl alcohol (4-hydroxy-3,5-dimethoxycinnamyl alcohol).
There are substantially two types of lignin on the market: sulfonate lignin and kraft lignin.
Kraft lignin is a by-product of the Kraft process, used to chemically extract cellulose from wood. It is obtained by precipitation, lowering the pH of the spent liquor of the Kraft process. For this reason kraft lignin, unlike sulfonate lignin, does not have sulfonate groups.
The inventors of the present invention have surprisingly found that the use of kraft lignin in the compound for production of the sealant layer is able to guarantee conservation of the sealing characteristics of the same during the life of the tyre.